The perception of distance is distinct from intelligibility. Sounds perceived as further away can be perfectly intelligible, but can be easily ignored. This circumstance is particularly important for all kinds of alarm tones and announcement signals that vitally require adequate attention. This is particularly true for announcement signals such as parking sensors and navigation signals in vehicles. Parking sensors are proximity sensors for road vehicles designed to alert the driver to obstacles while parking. These systems, which use either electromagnetic or ultrasonic sensors, are variously marketed by vehicle manufacturers under proprietary brand names such as Park Distance Control (PDC), Park Assist or Parktronic. In some applications, it is desirable to change the perceived distance of alarm signals.